Process for making wood laminate

ABSTRACT

A plywood adhesive especially useful for bonding hardwood overlays to softwood plywood cores which comprises a ureaformaldehyde adhesive modified with a phenol-formaldehyde resin.

United States Patent Horowitz et al.

[54] PROCESS FOR MAKING WOOD LAMINATE [72] inventors: Frederick HorowitzCoos Bay; Joseph B.

Dede, Jr., North Bend, both of Greg.

[73] Assignee: Georgia-Pacific Corporation, Portland,

Oreg.

[22] Filed: Apr. 9, 1969 211 App]. No.: 814,827

[52] US. Cl. ..l56/33l, 156/335, 161/270, 260/293, 260/32.4

[51] Int. Cl. ..C09j l/00 [58] Field ofSearch ..l56/33l,335;260/29.3,32.4; 161/270 Primary Examiner-Carl D. Quarforth AssistantExaminer-Stephen J. Lechert, Jr. Attorney-Peter P. Chevis [57] ABSTRACTA plywood adhesive especially useful for bonding hardwood overlays tosoftwood plywood cores which comprises a ureaformaldehyde adhesivemodified with a phenol-formaldehyde resin.

10 Claims, No Drawings PROCESS FOR MAKING WOOD LAMINATE This inventionpertains to an adhesive useful in wood lamination, especially in plywoodmanufacture for the bonding of thin overlays to softwood cores.Adhesives, for example for plywood preparation, must have certainworking characteristics in addition to the proper adhesive propertiesfor a satisfactory performance under modern production methods. Eventhough good strengths may be possible under certain conditions withparticular resins, the resins may not be suitable for plywood adhesives,if the working characteristics are such that these results cannot bereadily achieved under the accepted production methods employed inplywood manufacture.

in many processes, the adhesive is applied by spraying. Thus, inaddition to the requirements that the adhesive be fast setting, have agood pot life, and have the required penetration to form a strong bond,the consistency of the adhesive has to be such that it can be readilysprayable.

Further, in many operations, the assembled panels of plywood orlaminated wood products are pre-pressed cold prior to heat setting ofthe adhesive. By pre-pressing the assembled panels, the capacity of theheated platen presses is increased and the quality of the laminated woodproducts or plywood obtained can be materially improved. in coldprepressing, the adhesive must form a sufficient bond to permit thehandling of the pre-pressed panels without shifting or separation of theplys after the pressure is removed. The laminate or consolidated panelmay be stored for various lengths of time for from a few minutes tohours after prepressing before the panels are subjected to the hightemperature and pressure to finally set the adhesive. Thus, theadhesive, in addition to providing the necessary initial bond, mustremain after pre-pressing in a form capable of forming a strong finalbond upon heat setting.

Adhesives containing urea-formaldehyde resins which are light in colorare often used in overlay plywood'manufacture. However, thecharacteristics of the adhesives are such that the adhesives may not beapplied by spraying. Phenol-formaldehyde adhesives often used in plywoodoperations and wood laminate production are likewise not entirelysatisfactory for the overlay production. In the overlay production, theoverlay panel or top ply applied to the core is less than 1/16 inch inthickness, generally being l/2O to l/30 of an inch in thickness. It isusually of hardwood or a wood of light color. Due to the thinness andporosity of the top ply, the phenol-formaldehyde resin adhesives willbleed through the veneer imparting dark streaks in the finished panel.

It is, therefore, an object of this invention to provide an adhesive foruse in wood lamination. A further object is to provide an adhesive whichis especially applicable to thin veneers and has characteristicsenabling the adhesive to be applied by I spraying and used inpre-pressing operations. A still further object is to provide a processfor the lamination of wood using the above adhesive and the productthereof.

The above and other objects are attained, by this invention, by using aurea-formaldehyde adhesive composition which has been modified by theaddition of a phenol-formaldehyde resin and acidified to a pH in therange of 5 to 7. The phenol-formaldehyde resin is intermixed in anamount of from 10 to 90 weight percent of the urea-formaldehyde resinwith from 25 to 40 percent being preferred. In the preparation of theadhesive, the phenol-formaldehyde resin, pre-condensed under alkalineconditions to a particular degree, is intermixed with the extenders andother constituents such as fillers and accelerators, if used, prior tothe addition of urea-formaldehyde resin and acidification of theadhesive mixture. In so preparing the adhesive, a readily sprayable,fast setting product is obtained which will not bleed through the thinveneers.

Urea-formaldehyde resins generally used in wood laminating adhesive areused for the preparation of the adhesive of this invention. Prior to theaddition to the adhesive, the resins are usually prepared by thereaction of from 0.8 to 3.0 moles of formaldehyde per mole of urea underacidic conditions in an aqueous medium. The condensation of theurea-formaldehyde resin is advanced to the extent that theurea-formaldehyde resin, as a neutralized solution at a pH in the rangeof 7 to 10 containing about 65 weight percent resin, has a viscosity inthe range of about 200 to 1,500 centipoises, preferably 300 to 1,000centip'oises.

The phenol-formaldehyde resins used in the modification of theurea-formaldehyde adhesives are the alkaline catalyzed resins similar tothose used in plywood adhesives except that the resins are condensed toa lesser extent. The resins are usually prepared by reacting phenol with0.8 to 3.0 moles of formaldehyde per mole of phenol, preferably from 1.6to 2.5 moles of formaldehyde per mole of phenol, in an aqueous mediumunder alkaline conditions. The condensation is carried out only to theextent that the phenol-formaldehyde resin, as a solution at a pH in therange of 9 to 10 containing about 40 weight percent resin, has aviscosity in the range of IO to 200 centipoises, preferably in the rangeof 25 to l00 centipoises.

in addition to the resins, additives or other constituents ordinarilyused in the preparation of urea-formaldehyde adhesives for woodlamination such as plywood are usually employed in the preparation ofthe adhesives of this invention. These constituents are added in aboutthe same ratio, with respect to the resin, as that normally employed forurea-formaldehyde resins with the ratios being, however, based upon thecombined weight of the urea-formaldehyde and phenolformaldehyde resinsinstead of the urea-formaldehyde resin alone. Amylaceous extenders suchas wheat flour, starches, and tapioca are often used in amounts of from50 to 200 weight percent of the resins. Extenders of proteinaceousmaterial, such as soya flour or dried blood may also be added in amountsof from 5 to 25 weight percent of the resins. The protein extenders,especially dry blood, shorten the high temperature set time of theadhesive. Clays and other fillers and extenders, such as nutshell flourand agricultural residues, may likewise be added as well as acidicaccelerators, such as, for example, acid amine salts as aniline hydrogenchloride, and the like.

In the preparation of the adhesive, generally the extenders, fillers,and other constituents of the adhesive are intermixed with the alkalinephenol-formaldehyde resin solution as obtained upon pre-condensation ofthe phenol with the form aldehyde. The urea-formaldehyde resin is thenadded to the adhesive and the mixture acidified to a pH in the range of5 to 7 by the addition of organic or inorganic acids, or preferably,strong acidic salts such as the aluminum and ammonium salts of strongmineral acids such as sulfuric, hydrochloric, and nitric acids. Theadhesive may be acidified during or shortly before or after the additionof the urea-formaldehyde resin. Aluminum sulfate which is readilyavailable is most often used as the acidifying constituent. Generally,an amount of from 1 to 5 percent of aluminum sulfate, based upon theweight of the phenol-formaldehyde and urea-formaldehyde resins present,may be sufficient.

The adhesive thus prepared, using a phenol-formaldehyde resin condensedto a particular point, to modify the urea-formaldehyde adhesive will notbecome stringy as often is the case upon intermixing phenol-formaldehyderesin with ureaformaldehyde resin. The mixture usually has a viscosityin the range of 2,500 to 5,000 centipoises, maintains its consistency,and can be readily applied to wood laminates or plys by spraying. Thebleed through which is normally obtained with thin plys with thephenol-formaldehyde resins is not obtained and the adhesive issatisfactory for pre-pressing operations. Good bonds are obtained uponcold pressing and the final heat setting.

The following example further illustrates the invention.

EXAMPLE I In the preparation of a plywood panel with a hardwood veneeroverlay of about 1/20 of an inch in thickness, a phenolforrnaldehydeadhesive which was normally used was found to be unacceptable due to thebleed through" of the adhesive through the thin veneer.

An adhesive was prepared by intermixing, in 450 pounds of water, 25pounds of dried blood and 400 pounds of wheat flour with 140 pounds ofan aqueous 40 weight percent solution of phenol-formaldehyde resin. The40 weight percent solution of the phenol-formaldehyde resin at a pH of9.5 had a viscosity of 50 centipoises. The resin was prepared byreacting formaldehyde and phenol in the presence of sodium hydroxide ata mole ratio of 2 moles of formaldehyde per mole of phenol. Afterintermixing the above ingredients, 20 pounds of a 30 percent solution ofaluminum sulfate, papermakers alum grade, were added followed by 340pounds of a 65 weight percent solution of urea-formaldehyde resin. Theurea-formaldehyde resin had been prepared by. condensing 1.8 moles offormaldehyde per mole of urea. The adhesive mixture was then furtherdiluted with 150 pounds of water and was at a pH of 6.9. The adhesivehad a good pot life and was readily sprayable. A good bond was formedupon cold pressing for minutes so that the panel could easily be handledwithout shifting of the plys. It had a fast set time of about 3 minutesat 250 F and 175 pounds per square inch. With glue spreads ranging from45 to 65 pounds per. thousand square feet of double glue line, hardwoodoverlay plywood manufactured was tested by the Vacuum-Soak Test. Testspecimens (2 inch X 5 inch in size) were placed in a pressure vesseland'completely submerged in 1 F water. A vacuum of inches of mercury wasdrawn and maintained for 30 minutes and then released. Specimens werethen. allowed to soak in the same water at atmospheric pressure for 4 75hours with no additional heating. The specimens were then removed anddried for 15 hours at 150 F in an oven with forced-air circulation. Thetest pieces were then checked for continuous visible edge delaminationof V4 inch or more in depth and over two inches in length along any glueline. The panels met the standards set by the American PlywoodAssociation receiving ratings of from 95 to 100 percent. 7

Upon the elimination of the aluminum sulfate, bleeding of the adhesivethrough the thin veneer was obtained and the initial bonds obtained uponpre-pressing were poor. Further, after a short time, the mixture becamestringy and could not be properly dispersed by the use of a spray. Anadhesive employing only urea-formaldehyde resin was likewiseunsatisfactory. The mixture was gummy and could not be applied byspraying.

What is claimed is:

1. In a process for the manufacture of a wood laminate, wherein the woodlaminations are bonded by a urea-formaldehyde adhesive compositioncomprising urea-formaldehyde resin and an extender, the improvementwhich comprises using a modified urea-formaldehyde adhesive compositionin bonding said wood laminations, said modified urea-formaldehydeadhesive composition being an adhesive prepared by adding aphenol-formaldehyde resin to said urea-formaldehyde adhesive compositionin an amount of from 10 to 90 weight percent of the urea-formaldehyderesin in the adhesive and acidifying the adhesive containing thephenol-formaldehyde resin to a pH in the range of 5 to 7, saidphenol-formaldehyde resin being pre-condensed under alkaline conditionsuntil a 40 weight percent aqueous solution of the resin at a pH in therange of 8 to 9 has a viscosity in the range of 10 to 200 centipoises at25 C, and said phenol-formaldehyde resin being intermixed with theextender in the preparation of the densed until the 40 weight percentaqueous solution of the resin has a viscosity in the range of 25 to 100centipoises.

3. A process according to claim 2 wherein the adhesive mixture isacidified by the addition of a salt selected from the group consistingof aluminum and ammonium salts of sulfuric,

hydrochloric, and nitric acids.

. A process according to claim 3 wherein the salt is aluminum sulfate.

5. in a process for the manufacture of plywood, wherein the wood pliesare bonded by a urea-formaldehyde adhesive composition comprising aurea-formaldehyde resin and an extender, and wherein the adhesive isapplied by spraying, and wherein the surface ply is less than l/ 16 inchin thickness and the assembled panel prepressed prior to hot pressing,the improvement which comprises using a modified ureafonnaldehydeadhesive composition in bondingthe wood plies, said modifiedurea-formaldehyde adhesive composition being a urea-formaldehydeadhesive composition containing a phenol-formaldehyde resin in an amountof from 10 to weight percent of the urea-formaldehyde resin in theadhesive, and acidified with an acid salt to a pH in the range of 5 to7, said phenol-formaldehyde resin being condensed under alkalineconditions in a mole ratio of 0.8 to 3.0 moles of formaldehyde per moleof phenol until a 40 percent aqueous solution of. the resin has aviscosity in the range of from 10 to 200 centipoises at 25 C, and saidmodified urea-formaldehyde adhesive composition being prepared byintermixing the constituents in an order such that the acidification andthe addition of the urea-formaldehyde resin are made after the otherconstituents have been intermixed.

6. A process according to claims wherein the phenol-formaldehyde resinis pre-condensed under alkaline conditions in a mole ratio of from 1.6to 2.5 moles of formaldehyde per mole of phenol until the 40 weightpercent aqueous solution of the resin has a viscosity in the range of 25to centipoises.

7. A process according to claim 6 wherein the adhesive mixture isacidified by the addition of aluminum sulfate prior to the addition ofthe urea-formaldehyde resin to the adhesive mixture.

8. A process according to claim 7 wherein the amount ofphenol-formaldehyde resin added is in the range of from 25 to 40 percentof the urea-formaldehyde resin.

9. A process according to claim 7 wherein the modified urea-formaldehydeadhesive composition is prepared by intermixing the constituents in anorder such that the phenol-formaldehyde resin is intermixed withconstituents under alkaline conditions and subsequently theurea-formaldehyde resin is added and the adhesive acidified, saidurea-formaldehyde resin being pre-condensed in a mole ratio of 0.8 to3.0 moles of formaldehyde per mole of urea under acid conditions untilthe viscosity of a 65 weight percent aqueous solution of the resin has aviscosity in the range of from 100 to 1,500 centipoises at 25 C.

10. A process according to claim 9 wherein the urea-formaldehyde resinis pre-condensed until the viscosity of the 65 weight percent aqueoussolution is in the range of 300 to l ,000 centipoises. I

2. A process according to claim 1 wherein the phenol-formaldehyde resinis added in an amount of from 25 to 40 weight percent of theurea-formaldehyde resin and is pre-condensed until the 40 weight percentaqueous solution of the resin has a viscosity in the range of 25 to 100centipoises.
 3. A process according to claim 2 wherein the adhesivemixture is acidified by the addition of a salt selected from the groupconsisting of aluminum and ammonium salts of sulfuric, hydrochloric, andnitric acids.
 4. A process according to claim 3 wherein the salt isaluminum sulfate.
 5. In a process for the manufacture of plywood,wherein the wood plies are bonded by a urea-formaldehyde adhesivecomposition comprising a urea-formaldehyde resin and an extender, andwherein the adhesive is applied by spraying, and wherein the surface plyis less than 1/16 inch in thickness and the assembled panel prepressedprior to hot pressing, the improvement which comprises using a modifiedurea-formaldehyde adhesive composition in bonding the wood plies, saidmodified urea-formaldehyde adhesive composition being aurea-formaldehyde adhesive composition containing a phenol-formaldehyderesin in an amount of from 10 to 90 weight percent of theurea-formaldehyde resin in the adhesive, and acidified with an acid saltto a pH in the range of 5 to 7, said phenol-formaldehyde resin beingcondensed under alkaline conditions in a mole ratio of 0.8 to 3.0 molesof formaldehyde per mole of phenol until a 40 percent aqueous solutionof the resin has a viscosity in the range of from 10 to 200 centipoisesat 25* C, and said modified urea-formaldehyde adhesive composition beingprepared by intermixing the constituents in an order such that theacidification and the addition of the urea-formaldehyde resin are madeafter the other constituents have been intermixed.
 6. A processaccording to claim 5 wherein the phenol-formaldehyde resin ispre-condensed under alkaline conditions in a mole ratio of from 1.6 to2.5 moles of formaldehyde per mole of phenol until the 40 weight percentaqueous solution of the resin has a viscosity in the range of 25 to 100centipoises.
 7. A process according to claim 6 wherein the adhesivemixture is acidified by the addition of aluminum sulfate prior to theaddition of the urea-formaldehyde resin to the adhesive mixture.
 8. Aprocess according to claim 7 wherein the amount of phenol-formaldehyderesin added is in the range of from 25 to 40 percent of theurea-formaldehydE resin.
 9. A process according to claim 7 wherein themodified urea-formaldehyde adhesive composition is prepared byintermixing the constituents in an order such that thephenol-formaldehyde resin is intermixed with constituents under alkalineconditions and subsequently the urea-formaldehyde resin is added and theadhesive acidified, said urea-formaldehyde resin being pre-condensed ina mole ratio of 0.8 to 3.0 moles of formaldehyde per mole of urea underacid conditions until the viscosity of a 65 weight percent aqueoussolution of the resin has a viscosity in the range of from 100 to 1,500centipoises at 25* C.
 10. A process according to claim 9 wherein theurea-formaldehyde resin is pre-condensed until the viscosity of the 65weight percent aqueous solution is in the range of 300 to 1,000centipoises.